Lithographic apparatus and device manufacturing method

ABSTRACT

An off-axis alignment system in a lithographic projection apparatus uses broadband radiation to illuminate a phase grating on the wafer. The broadband radiation source may include fluorescent materials, e.g. Yag:Ce or ND:Yag crystals illuminated by excitation light.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a lithographic projectionapparatus.

[0003] 2. Description of the Related Art

[0004] The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0005] Another example of a pattering device is a programmable mirrorarray. One example of such an array is a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate filter, the undiffracted light can be filtered outof the reflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing piezoelectricactuators. Once again, the mirrors are matrix-addressable, such thataddressed mirrors will reflect an incoming radiation beam in a differentdirection to unaddressed mirrors. In this manner, the reflected beam ispatterned according to the addressing pattern of the matrix-addressablemirrors. The required matrix addressing can be performed using suitableelectronics. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be seen, forexample, from United States patents U.S. Pat. Nos. 5,296,891 and5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the caseof a programmable mirror array, the support structure may be embodied asa frame or table, for example, which may be fixed or movable asrequired.

[0006] Another example of a pattering device is a programmable LCDarray. An example of such a construction is given in U.S. Pat. No.5,229,872. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0007] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

[0008] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once. Such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus, commonlyreferred to as a step-and-scan apparatus, each target portion isirradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction. Since, in general, the projectionsystem will have a magnification factor M (generally <1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be seen, for example, fromU.S. Pat. No. 6,046,792.

[0009] In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake

[0010] B), development, a hard bake and measurement/inspection of theimaged features. This array of procedures is used as a basis to patternan individual layer of a device, e.g. an IC. Such a patterned layer maythen undergo various processes such as etching, ion-implantation(doping), metallization, oxidation, chemo-mechanical polishing, etc.,all intended to finish off an individual layer. If several layers arerequired, then the whole procedure, or a variant thereof, will have tobe repeated for each new layer. Eventually, an array of devices will bepresent on the substrate (wafer). These devices are then separated fromone another by a technique such as dicing or sawing, whence theindividual devices can be mounted on a carrier, connected to pins, etc.Further information regarding such processes can be obtained, forexample, from the book “Microchip Fabrication: A Practical Guide toSemiconductor Processing”, Third Edition, by Peter van Zant, McGraw HillPublishing Co., 1997, ISBN 0-07-067250-4.

[0011] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791.

[0012] The ever present demand in lithography to be able to image maskpatterns with ever decreasing critical dimension (CD) necessitatesincreasing overlay accuracy (the accuracy with which two successivelayers can be aligned with respect to each other). This drives a needfor ever increasing alignment accuracy. Since the overlay error must bemuch smaller than the critical dimension and the alignment error is notthe only contribution to overlay error, a critical dimension of 90 nmdemands an alignment accuracy of 10 nm or less.

[0013] A known through-the-lens (TTL) alignment system uses linear phasegratings of 16 μm pitch etched onto the substrate which are illuminatedby laser light. The diffracted light is then imaged on a referencegrating. By scanning the substrate underneath the alignment system anddetecting the light passing through the reference grating as a functionof stage position, the position of the substrate can be estimated withnanometer accuracy. However, the known TTL alignment system uses onewavelength of laser light and is subject to process dependent errors.Such errors occur when previously produced process layers formdiffractive structures affecting the wavelengths used in the alignmentsystem. An alignment system using one wavelength of light is stronglyaffected by such errors, introducing a second frequency reduces theseerrors somewhat by averaging, since the different wavelengths will notbe affected in the same way, but does not eliminate them entirely. Sucherrors can also be caused by asymmetrically deformed alignment marks.

[0014] U.S. Pat. No. 5,371,570 discloses a through the lens alignmentsystem using broadband radiation to illuminate alignment marks on thewafer. However, the alignment radiation is produced by a halogen lamp.The beam produced by such a lamp has a high étendue (solid anglesubtended by the beam multiplied by the area of the cross-section of thebeam) therefore it is difficult to obtain a high measurement lightintensity at the alignment mark, resulting in a low signal to noiseratio (SNR).

[0015] WO 98/39689 discloses an off-axis alignment system that usesmultiple wavelengths and higher diffraction orders to avoid errorsresulting from asymmetry of the alignment mark caused bychemical-mechanical polishing. The image of the grating is imaged foreach color on a different reference grating to obtain a measurementsignal.

[0016] U.S. Pat. No. 5,559,601 discloses an alignment system that useslaser diodes, e.g. providing four wavelengths, to illuminate mask andwafer marks. The wafer is scanned relative to the mask and alignmentinformation derived by Fourier analysis of the intensity of the returnradiation as a function of wafer position.

SUMMARY OF THE INVENTION

[0017] It is an aspect of the present invention to provide an improvedalignment system, in particular one which is less susceptible toprocess-dependent effects.

[0018] This and other aspects are achieved according to the invention ina lithographic apparatus including a radiation system constructed andarranged to provide a projection beam of radiation; a support structureconstructed and arranged to support a patterning device, the patterningdevice constructed and arranged to pattern the projection beam accordingto a desired pattern; a substrate table that holds a substrate; aprojection system constructed and arranged to project the patterned beamonto a target portion of the substrate; and an off-axis alignment systemincluding a radiation source constructed and arranged to illuminate aphase grating on a substrate held on the substrate table and an imagingsystem constructed and arranged to image diffracted light from the phasegrating onto an image plane, wherein the imaging system images the phasegrating onto one single image plane substantially correctly at at leasttwo distinct wavelengths.

[0019] The use of an imaging system capable of imaging at least twowavelengths correctly onto a single image plane is advantageous in thatit is more robust than single wavelength alignment. The use of multiplecolors effectively averages out certain errors in the alignment signaldue to asymmetric marks and the detection at one single imaging planemakes it unnecessary to mix the detection single of the two distinctwavelengths. Furthermore it diminishes the effect of thin filminterference effects on signal strength. Both a true broadband spectrumand a set of discrete (laser) wavelengths may be used.

[0020] An advantage of periodic structures (gratings) over non-gratingmark types is that effectively only a part of the total NA of theimaging system is used because the light is diffracted in verydistinctively determined orders by the grating. By using non-gratingmark types the mark image will be equally distributed over the total NAof the imaging system, and will be equally sensitive to aberrations inthe total area of the pupil. The effective area of the pupil that isbeing used is also determined by the NA of the illumination system,however that is of relatively small influence.

[0021] The alignment system may comprise an illumination system forilluminating the phase grating with an NA greater than 0.01 preferablygreater than 0.1 and most preferably greater than about 0.2. The use ofan illumination NA larger than 0.01 is advantageously to get enoughlight on the grating, which is a problem for broadband sources having ahigh etendue. Laser sources commonly used for illuminating purposes havea low etendue and therefore there is no need for illuminating with a NAhigher than 0.01 to get enough light upon the grating. Just the planewave of the laser is radiated upon the grating.

[0022] Another advantage of illuminating with a relative high NA is thatthis makes the system less sensitive to illumination angle dependenterrors. If the grating is illuminated from one direction, all theradiation from that one direction may suffer from the same illuminationangle dependent error so that the total alignment signal is dependent onthat error. In a higher NA illumination system the radiation isdistributed over different illumination angles so that the illuminationangle dependent errors are averaged out for the different angles.

[0023] A drawback of the relatively high illumination NA is that thegrating must be in the focal plane of the illumination beam. The highillumination NA and the grating period make it further necessary thatthe imaging system for projecting diffracted light from the phasegrating on the reference grating needs a relatively high NA. The imagingsystem may have a NA greater than about 0.7, preferably greater than0.8, most preferably greater than about 0.9. The High NA of the imagingsystem makes also the imaging system focus sensitive. A separatefocussing sensor is therefore needed in the alignment system.

[0024] Another drawback of the illumination system having a high NA isthat the radiation of the illumination must have an homogeneous angulardistribution. The use of a specially designed homogenizer may thereforebe necessary.

[0025] The use of a small, e.g. less than 5 μm, preferably 1 μm, pitchgrating enables a reduction of the interpolation needed in the dataanalysis/position estimation. This will proportionally decrease theinfluence of noise and mark asymmetry on the aligned position.Furthermore it enables the total area of the mark to be made as small aspossible. Since the averaging of the alignment signal over the siliconarea is actually related to the number of “edges” that are present inthe alignment mark, the averaging and thus insensitivity for localperturbations is increased by decreasing the period of the phasegrating.

[0026] According to a further aspect of the invention there is provideda device manufacturing method including providing a substrate that is atleast partially covered by a layer of radiation-sensitive material;providing a projection beam of radiation using a radiation system; usinga patterning device to endow the projection beam with a pattern in itscross-section; projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material; aligning thesubstrate to a reference grating by illuminating a phase gratingprovided on the substrate with radiation and imaging diffracted lightfrom the phase grating onto the reference grating using an imagingsystem arranged to image the phase grating onto the reference gratingsubstantially correctly at at least two distinct wavelengths.

[0027] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

[0028] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or126 nm) and EUV (extreme ultra-violet radiation, e.g. having awavelength in the range 5-20 nm), as well as particle beams, such as ionbeams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINS

[0029] Embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich:

[0030]FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the invention;

[0031]FIG. 2 is a diagram of the imaging section of an alignment systemaccording to a first embodiment of the invention;

[0032]FIG. 3 is a diagram of the detection section of the alignmentsystem according to the first embodiment of the invention;

[0033]FIG. 4 is a diagram of a quad cell sensor used in the alignmentsystem according to the first embodiment of the invention;

[0034]FIGS. 5 and 6 are diagrams used in explaining detection of correctfocus in the alignment system;

[0035]FIG. 7 is a diagram of a homogenizer used in the alignment systemaccording to the first embodiment of the invention; and

[0036]FIGS. 8 and 9 are diagrams of two alternative light sources usablein the first embodiment of the invention.

[0037] In the Figures, corresponding reference symbols indicatecorresponding parts.

DETAILED DESCRIPTION

[0038]FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes a radiation system Ex, IL that supplies a projection beam PB ofradiation (e.g. UV or EUV radiation). In this embodiment, the radiationsystem also comprises a radiation source LA; a first object table (masktable) MT provided with a mask holder for holding a mask MA (e.g. areticle), and connected to a first positioning device M₁, M₂ toaccurately position the mask with respect to a projection system PL; asecond object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to a second positioning device P₁, P₂ to accuratelyposition the substrate with respect to the projection system PL; theprojection system (“lens”) PL (e.g. a refractive or catadioptric system,a mirror group or an array of field deflectors) constructed and arrangedto image an irradiated portion of the mask MA onto a target portion C(e.g. comprising one or more dies) of the substrate W. The projectionsystem PL is supported on a reference frame RF. As here depicted, theapparatus is of a transmissive type (i.e. has a transmissive mask).However, in general, it may also be of a reflective type, for example(with a reflective mask). Alternatively, the apparatus may employanother kind of patterning device, such as a programmable mirror arrayof a type as referred to above.

[0039] The source LA (e.g. an excimer laser, an undulator or wigglerprovided around the path of an electron beam in a storage ring orsynchrotron, a laser-produced plasma source, a discharge source or anelectron or ion beam source) produces a beam PB of radiation. The beamPB is fed into an illumination system (illuminator) IL, either directlyor after having traversed a conditioner, such as a beam expander Ex, forexample. The illuminator IL may comprise an adjusting device AM forsetting the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in thebeam. In addition, it will generally comprise various other components,such as an integrator IN and a condenser CO. In this way, the beam PBimpinging on the mask MA has a desired uniformity and intensitydistribution in its cross-section.

[0040] It should be noted with regard to FIG. 1 that the source LA maybe within the housing of the lithographic projection apparatus (as isoften the case when the source LA is a mercury lamp, for example), butthat it may also be remote from the lithographic projection apparatus,the radiation beam which it produces being led into the apparatus (e.g.with the aid of suitable directing mirrors). The latter scenario isoften the case when the source LA is an excimer laser. The currentinvention encompasses both of these scenarios.

[0041] The beam PB subsequently intercepts the mask MA, which is held onthe mask table MT. Having traversed the mask MA, the beam PB passesthrough the lens PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the second positioning device P₁, P₂(and interferometer IF), the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of thebeam PB. Similarly, the first positioning device M₁, M₂ can be used toaccurately position the mask MA with respect to the path of the beam PB,e.g. after mechanical retrieval of the mask MA from a mask library, orduring a scan. In general, movement of the object tables MT, WT will berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning). However, in the case of a waferstepper (as opposed to a step-and-scan apparatus) the mask table MT mayjust be connected to a short stroke actuator, or may be fixed.

[0042] The depicted apparatus can be used in two different modes:

[0043] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at once (i.e. a single“flash”) onto a target portion C. The substrate table WT is then shiftedin the x and/or y directions so that a different target portion C can beirradiated by the beam PB;

[0044] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash”.Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g. the y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image.Concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0045] An off-axis alignment system forming part of the first embodimentof the present invention is shown generally in FIG. 2. The alignmentsystem 1 comprises source module 2, optical module or imaging system 3and detection module 4.

[0046] Source module 2 comprises a broadband source 21 with a lowétendue, which is described further below, which outputs broadbandradiation, e.g. in the visible region, into a multi-mode fiber 22.Interposed in the multi-mode fiber 22 is a homogenizer 23, alsodescribed further below. The output end of multi-mode fiber 22 is heldin bracket 24 which also mounts lens 25. Lens 25 feeds the illuminationlight into the illumination branch 31 of the optical module 3. Theillumination branch 31 comprises lenses 312, 313 which, together withlens 25 of the source module 2, focus the output facet of the fiber witha magnification of about 5 onto a small 45° mirror 315 which folds thebeam into the imaging branch 32 of the optical module 3. Mirrors 311 and314 are provided in the illumination branch 3 for convenient folding ofthe beam. Bracket 24 allows the end of fiber 22 and lens 25 to bepositioned in three dimensions for accurate positioning of the sourceimage.

[0047] Starting from the bottom, the imaging branch 32 comprises a highnumerical aperture (NA), long working distance microscope objective 320.Next are two field lenses 319, 318 that re-image the wafer W onto thefirst intermediate image plane at which field stop 317 is provided.Lenses 318, 319 are arranged such that the first part of the imagingsystem is telecentric on both image and object side, with amagnification of exactly 30. At a pupil plane, spatial filter 321 isprovided. The filter 321 has an opaque center 321 b and apertures 321 aextending parallel to the X and Y directions to select only the ordersdiffracted in the X and Y directions, i.e. not those diffracted by thediagonal structures in the mark and not the 0^(th)-order. This part ofthe imaging system is telecentric on the object side (field stop 317)but not the image side, where a reference plate or mark or grating 324is provided. This enables the overall length of the system to bereduced. The lenses 322, 323 are selected and positioned so that thetotal magnification of the imaging system from the wafer to the plane ofthe reference plate 324 is exactly 50. The magnification of the secondpart of the imaging branch is therefore 1⅔.

[0048] It will be appreciated that the magnification of the imagingsystem is related to the pitches of the substrate mark and referencegrating. Because the 0^(th)-order is blocked, the pitch of the substratemask P_(substrate), the magnification M and the pitch of the referencegrating P_(ref) must satisfy the following equation:$P_{ref} = {\frac{1}{2} \cdot M \cdot P_{substrate}}$

[0049] The components of the optical module 3 are preferably rigidlymounted to a frame 33 made of an ultra low expansion material such asInvar or Zerodur and mounted on the reference frame of the apparatus.

[0050] The microscope objective 320 forms the first lens of the imagingbranch of the optical module. This lens must have a numerical aperturelarge enough to capture sufficient diffraction orders from the alignmentmark on the wafer and may, for example, have a numerical aperture NA ofat least 0.8 or 0.9. It is additionally preferred to have a reasonabledistance between the wafer and alignment system so that a long workingdistance objective is preferred. Commercially available microscopeobjectives are usable. The arrangement illustrated in FIG. 2 makes useof a microscope objective that does not have an accessible pupil plane.Accordingly, lenses 318, 319, 316 are provided to re-image the pupilplane at a physically accessible location where a pupil stop 321 can beprovided. A more compact arrangement may be obtained if a microscopeobjective having a physically accessible pupil plane is used. Suitableobjectives are known for use in phase contrast microscopes.

[0051] As will be appreciated, the basic principle of the alignmentsystem is that an alignment mark provided on the wafer is imaged onto acorresponding reference mark provided in the system and alignmentinformation is derived from measuring the intensity of radiation passingthrough the reference grating as the wafer is scanned. In the presentinvention, the reference mark comprises a two-dimensional grating havingdiamond-shaped unit cells, as shown in the enlargement in FIG. 2. Thereference mark 324 is arranged to be symmetric around the optical axisof the imaging branch of the alignment system. This symmetry suppressesthe influence of chromatic magnification errors on the aligned position.Since a change of magnification causes symmetric distortions, the errorson both sides of the optical axis cancel each other out, at least forsmall magnification errors. The use of a two-dimensional grating enablesdetection of alignment in both X and Y directions whilst preservingcomplete symmetry around the optical axis. Note though that thealignment marks on the wafer are still linear gratings and that only onedirection is measured at a time.

[0052] The field stop 317 is positioned at the first intermediate imageof the wafer and thereby serves as the field stop for both illuminationand imaging. The imaging field can be further reduced by placing anadditional field stop at the position of reference mark 324. To minimizethe effects of clipping of the field, the field aperture 317 a iscircular. Acting as field stop of the imaging system, the field stop 317determines the area of the mark that is detected. In the presentinvention, the detection field is smaller than the total mark size sothat the detection field can remain within the mark during the scan ofthe alignment mark. This means there is no envelope in the intensity ofthe alignment signal, improving fitting in the detection system. Actingas field stop for the illumination branch, field stop 317 limits thefield of illumination to be only slightly larger or identical to thedetection field. This avoids the possibility that structures adjacentthe alignment mark are also illuminated, which might lead to straydiffraction entering the imaging system and causing errors in thealignment signal.

[0053] Detection module 4 primarily measures the intensity of the lighttransmitted through the reference mark 324 that is located in an imageplane of the system. The detection module also detects the focus signaland provides camera images of both the pupil plane and the wafer plane.The detection module 4 is shown in more detail in FIG. 3.

[0054] The main signal detection branch 41 of the detection module 4comprises lenses 411, 412 which image the circular field of thealignment system on the center of a quad cell 413. The quad cell 413 hasfour sections 413 a, b, c, d (shown in FIG. 4) so that four differentpoints (shown by open circles) in the field can be measured. Each cellof the quad cell 413 is a silicon photodiode. The intensity detected bythe cells of the quad cell is a sine function of substrate tableposition with which alignment can be carried out in a known manner. Theexact position of the effective measurement point is dependent on theintensity distribution over the field and in general the layout of thephotodiode and the shape of the field. Measuring at four pointssimultaneously provides advantages that the relative magnification androtation of the reference grating with respect to the wafer grating caneasily be determined from one alignment scan. This enables rapid initialalignment of the module and long-term monitoring of the performance ofthe alignment system.

[0055] A second, optional, signal detection branch 43 comprises ahalf-silvered mirror 431 to divert a proportion of the detection beamand a lens 432 which gathers the light and couples it into a multi-modefiber 433 which transports it to a photo-multiplier tube 434conveniently arranged in the electronics module of the apparatus. Thephoto-multiplier is used for detection of very weak alignment markssince it can do shot noise limited detection and noiseless amplificationof the alignment signal.

[0056] Camera branch 42 comprises beams splitters 421, 422 and 425 aswell as lens 423 which divert light onto CCD cameras 424, 426 placedrespectively at image and pupil planes of the detection module and tosplit detector 427.

[0057] Split detector 427 is placed in a pupil plane of the referencegrating 324. In this plane, there will be diffraction spots at aseparation determined by the period of the substrate and referencegratings and a size determined by the aperture of the imaging system 3.If the imaging system 3 is in focus, i.e. the substrate and referencegratings are in conjugate planes, the intensity distribution in thespots will be homogeneous. However, defocus will cause inhomogeneities.This is shown in the graph of FIG. 5 which shows intensity with xposition in the pupil plane. Horizontal straight line a is for a systemin correct focus, inclined straight line b is for a system with slightdefocus and sinusoidal curve c is for a system with a larger degree ofdefocus. If the gratings are scanned in the x-direction, the intensityprofile will show a phase shift between the two halves of thephotodetector, if the system is out of focus.

[0058] This arrangement can also be used with a detector divided into agreater number of segments. The above method of detecting defocusdepends on height to the diffraction grating on the substrate and henceis not affected by subsequent process layers.

[0059] An alternative way to detect the focus signal makes use of thefact that the apparent aligned position is dependent on the angle ofillumination of the alignment mark when it is not properly focussed. Asplit detector placed in the image pupil of the alignment system afterthe reference grating enables the apparent aligned position to bemeasured separately using beams that have a positive angle of incidenceand beams that have a negative angle of incidence. The difference inapparent aligned position therefore indicates the degree of defocus.

[0060] It should be noted that the alignment signal is taken from thefirst orders coming from the reference grating and accordingly theseorders are isolated from the remainder of the light by a pupil filter(not shown) provided in the pupil plane of the detection module.

[0061] The above alignment system is designed to receive light via amulti-mode fiber 22 and can use light in a broad wavelength range sothat many different forms of light source 21 can be used. The sourceshould have a range of wavelengths, a set of discrete, spaced apartwavelengths or a variable wavelength, e.g. in the range of from 500 to700 nm, and at the output of the fiber 22 should have a homogeneousspatial as well as angular distribution. Additionally, the light ispreferably modulated, e.g. at 50 kHz, in a known manner to enablesynchronous detection. Possible sources are listed in the table below.Source Wavelength (nm) Xe-Arc Lamp 500-700 LED 680 He—Ne Laser 632.8D-Nd-Yag Laser 532 Laser Diode 640

[0062] To provide the desired angular homogeneity, homogenizer 23 isprovided in multi-mode fiber 22 which brings the illumination light fromsource 21. The multi-mode fiber 22 provides sufficient spatialhomogeneity but retains any angular inhomogeneity of the source even fora 5 m long fiber. Homogenizer 23, as shown in FIG. 7, comprises lenses231, 232 arranged such that fiber entrance of the output fiber 22 b islocated in the pupil of the optical system formed by lenses 231, 232.This effectively swaps the spatial and angular coordinates so that boththe angular and spatial coordinates are homogenized by the two sections22 a, 22 b of the multi-mode fiber 22 without introducing significantlosses.

[0063] A particularly preferred radiation source 21 is shown in FIG. 8.Radiation source 21 comprises a laser or laser diode 211 emitting lighthaving a wavelength in the blue region of the spectrum. The blue lightis focussed by lenses 212, 213 on a fluorescent crystal 214. This maycomprise Yag:Ce or ND:Yag crystals or the like. These crystals, whenexcited by blue wavelength radiation, emit fluorescent light with abroad band of wavelengths. If the crystals are not in a laser cavity,the emission is isotropic over the total space of the channel filledwith the crystal which can be made small such that some radiation islocked into the channel which acts as a multi-mode wave guide. Radiationthen leaves the channel on one side forming a source with a highintensity and a low étendue. The radiation is coupled into fiber 22 bylens 215. The size of the channel is dependent on the size of thecrystals and may, for example, have a 100 μm cross-section. The walls ofthe channels may be transmissive or reflective, a reflective wall givinga higher efficiency. The color of the output can be adjusted by mixingdifferent types of crystal.

[0064]FIG. 9 shows a variant light source 21′ in which the fluorescentcrystals 214′ are mounted on a reflective substrate 217. A beam splitter216 is provided to direct the output, via collimating lens 215, intomulti-mode fiber 22.

[0065] While specific embodiments of the invention have been describedabove, it will be appreciated that the invention may be practicedotherwise than as described. The description is not intended to limitthe invention. Although the invention has been described in relation toalignment of a substrate to a reference grating, it may also be used foralignment of other objects, e.g. a mask.

1. A lithographic projection apparatus, comprising: a radiation systemconstructed and arranged to provide a projection beam of radiation; asupport structure constructed and arranged to support a patterningdevice, the patterning device constructed and arranged to pattern theprojection beam according to a desired pattern; a substrate table thatholds a substrate; a projection system constructed and arranged toproject the patterned beam onto a target portion of the substrate; andan off-axis alignment system comprising a radiation source constructedand arranged to illuminate a phase grating on a substrate held on thesubstrate table and an imaging system for imaging diffracted light fromthe phase grating onto an image plane, wherein the imaging system imagesthe phase grating onto one single image plane substantially correctly atat least two distinct wavelengths.
 2. An apparatus according to claim 1,wherein the alignment system comprises an illumination systemconstructed and arranged to illuminate the phase grating with a numericaperture NA greater than about 0.01.
 3. An apparatus according to claim1, wherein the alignment system comprises an illumination systemconstructed and arranged to illuminate the phase grating with a numericaperture NA greater than about 0.1.
 4. An apparatus according to claim1, wherein the alignment system comprises an illumination systemconstructed and arranged to illuminate the phase grating with a numericaperture NA greater than about 0.2.
 5. An apparatus according to claim1, wherein the imaging system has a numeric aperture NA greater thanabout 0.7.
 6. An apparatus according to claim 1, wherein the imagingsystem has a numeric aperture NA greater than about 0.8.
 7. An apparatusaccording to claim 1, wherein the imaging system has a numeric apertureNA greater than about 0.9.
 8. An apparatus according to any claim 1,wherein the alignment system comprises an illumination systemconstructed and arranged to illuminate the phase grating with radiationhaving homogeneous spatial and angular distributions.
 9. Apparatusaccording to claim 7, further comprising a homogenizer interposed in afiber which couples the radiation source to the illumination system, thehomogenizer comprising an optical system constructed and arranged tocouple radiation from an exit face of a first part of the fiber into anentrance face of a second part of the fiber, the exit face of the firstpart of the fiber being positioned near to an object plane of theoptical system and the entrance face of the second part of the fiber isnear a pupil plane of the optical system.
 10. An apparatus according toany claim 1, wherein the radiation source comprises at least two sourceseach emitting substantially monochromatic radiation.
 11. An apparatusaccording to claim 1, wherein the radiation source emits wavelengthsover a substantially continuous range of wavelengths, the range ofwavelengths covering at least 50 nm.
 12. An pparatus according to claim11, wherein the radiation source comprises a fluorescent material and anexcitation source constructed and arranged to direct excitationradiation onto the fluorescent material.
 13. Apparatus according toclaim 12, wherein the fluorescent material is provided in a channelforming a multi-mode waveguide.
 14. A device manufacturing method,comprising: providing a substrate that is at least partially covered bya layer of radiation-sensitive material; providing a projection beam ofradiation using a radiation system; using a patterning device to endowthe projection beam with a pattern in its cross-section; projecting thepatterned beam of radiation onto a target portion of the layer ofradiation-sensitive material, aligning the substrate to a referencegrating by illuminating a phase grating provided on the substrate withradiation and imaging diffracted light from the phase grating onto thereference grating using an imaging system arranged to image the phasegrating onto the reference grating substantially correctly at at leasttwo distinct wavelengths.